There are only relatively few solvents capable of dissolving cellulose physically (i.e. without intervening chemical derivatization as in the viscose process). They include alkylpyridinium salts (U.S. Pat. No. 1,943,176). However, alkylpyridinium salts generally have a melting point of more than 100° C. Moreover, in the solution, significant degradation of the cellulose chains occurs in that the average degree of polymerization (DP) of the cellulose decreases. This is a disadvantage of this process.
US 2010/0305249 describes homogeneous solutions of cellulose in tetraalkylammonium alkylphosphates. The alkyl groups may be the same or different. They are each preferably straight-chain or branched C1-C5 alkyl groups or C2-C20 alkoxy groups. Tributylmethylammonium dimethylphosphate, tributylethylammonium diethylphosphate, tripropylmethylammonium dimethylphosphate and tripropylethylammonium diethylphosphate are particularly preferred tetraalkylammonium alkylphosphates. The solutions may contain cosolvents in the form of protic or aprotic solvents or ionic liquids (other than the tetraalkylammonium alkylphosphates mentioned), such as hexamethylphosphoramide, N-methylpyrrolidone, nitromethane, N,N-dimethylformamide, N,N-dimethylacetamide, acetonitrile, sulpholane dimethylsulphoxide, aliphatic carboxylic acids, amines or imidazolium salts. The amount of cosolvents is generally from 1 to 15 wt %, based on the total weight of all solvents. The amount of cellulose is generally about 1 to 40 wt %, based on the total weight of the solution. The cellulose can be chemically modified in these solutions in that, for example, it can be converted into cellulose acetate by reaction with acetic anhydride.
The cellulose solutions described in US 2012/0041080 A1 comprise the same tetraalkylammonium alkylphosphates as the solutions described in US 2010/0305249 except that the alkyl groups are straight-chain or branched C1-C5 alkyl groups. Alkoxy groups are not mentioned. The same cosolvents can be used. However the amount of cosolvents is 5 to 90 wt %, based on the total weight of all solvents.
Also known are solutions of cellulose in an ionic liquid where the cation derives from a strong organic base, such as 1,1,3,3-tetramethylguanidine, 1,1,2,3,3-pentamethylguanidine, 1,8-diazabicyclo[5.4.0]undec-7-ene, 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine or iminotris(dimethylamino)phosphorane, and the anion derives from a Brönstedt acid, such as propionic acid, hydrochloric acid, methyl dihydrogenphosphonate or phosphinic acid (WO 2011/161326 A2).
Triethylmethylammonium formate is likewise known to be a useful solvent for cellulose (S. Köhler, T. Liebert, T. Heinze, Ammonium-based Cellulose Solvents Suitable for Homogeneous Etherification, Macromol. Biosci. 9 [2009] 836-841). However, in 155° C. it has a relatively high melting point, which is unacceptable for many reactions on cellulose. Adding aqueous formic acid was the only way to adjust the melting point, but greatly limits the possible reactions in this system. Furthermore, derivatizatien of the cellulose occurs during dissolution in that cellulose formate is produced. The maximum cellulose concentration in these solutions is 10 wt %.
Solutions of cellulose in ionic liquids generally display a very high level of viscosity. Moreover, many ionic solvents for cellulose have a relatively high melting point (distinctly above 100° C.). Some degradation of the polymer chains of the cellulose is often observed at the dissolution temperatures needed. Moreover, ionic liquids are relatively costly and have only limited recoverability for reuse. Notably ionic liquids based on heterocycles have a tendency, in homogeneous reactions on the cellulose, to undergo secondary reactions such as ring opening or nucleophilic substitution.